Editor Profile: Pascal Genschik

Pascal Genschik

Accounting for more than 1600 genes in Arabidopsis (Arabidopsis thaliana), the ubiquitin-proteasome system (UPS) plays a central role in nearly all facets of plant biology ranging from hormone biology and plant pathogen responses to development and epigenetics (Vierstra, 2009). Pascal Genschik has devoted much of his research career to disentangling the complexity of this degradation system.

Genschik’s body of work has kept pace with the reach of the UPS and explored topics as wide ranging as ethylene signaling, viral pathogenesis, and cell cycle regulation, to name a few.

THE EARLY PATH TO DISCOVERY

Believing that there are still many places yet to be discovered, Genschik was drawn to the life of an explorer from a young age. A mammoth tooth—gifted to him by a paleontology enthusiast neighbor—fueled his imagination and stimulated his early interests in prehistory and fossil collecting. Although this served to pique his interest in biology and history classes, Genschik was largely uninspired by school. Instead, he would indulge his pioneering adolescent spirit by becoming an avid reader of science fiction and adventure novels.

At the University of Strasbourg, Genschik majored in biology and naturally gravitated toward courses in evolution, but he also found himself excelling in cell biology and genetics. Recognizing that biotechnology and biomedicine could solve many societal problems, he made the fortuitous decision to pursue masters and doctoral training in Dr. Jacqueline Fleck’s laboratory to build a foundation in molecular biology. Not only would this serve as his portal into plant biology but also it would become his gateway into the world of ubiquitin and the proteasome.

As a graduate student, Genschik focused on identifying components of the de-differentiation machinery in Arabidopsis and Nicotiana. At the time, little was known about how plants can achieve immense levels of plasticity that enable them to reprogram to a stem cell state from which they can differentiate anew—a property that permits many plants to regrow from cuttings without supplementary treatments. Genschik cloned and characterized ubiquitin/proteasome system genes that were induced early during the de-differentiation process (Genschik et al. 1990, 1992a, 1992b, 1994a, 1994b, 1994c). His work yielded six first-author publications and the Adrerus PhD Prize for Excellence from the University of Strasbourg in recognition of his prolific research accomplishments.

THE RNA WORLD

For his postdoctoral research, Genschik shifted focus to RNA biology in Professor Witold Filipowicz’s laboratory at the Friedrich Miescher Institute in Switzerland. Genschik used biochemical and molecular approaches to understand how the 3′ end of some RNA molecules is cyclically modified. Whereas these cyclized ends are most commonly generated by RNases following cleavage, RNA 3′-terminal phosphate cyclases enable their formation in the absence of a hydrolysis event. Genschik’s research cloned the first RNA 3′-terminal phosphate cyclase and further refined the mechanism by which it catalyzes these reactions (Genschik et al., 1997a). Despite their conservation in archaea, bacteria, and eukarya, the precise cellular roles of RNA 3′-terminal phosphate cyclases in RNA processing are still under active investigation in several laboratories worldwide.

Genschik’s work on RNA 3′-terminal phosphate cyclases, but also on cyclic phosphodiesterases, culminated in three first-author publications, each focusing on vastly different models including HeLa cells, Escherichia coli, and Arabidopsis (Genschik et al., 1997a, 1997b, 1998a). Genschik credits his productive postdoctoral research years to his mentor Witold Filipowicz for defining the meaning of excellence in science through his enthusiasm and incredibly broad knowledge of biology.

FINDING A NICHE

Genschik’s time in Switzerland was also transformative for rekindling his interest in ubiquitin. Prominent lectures by leaders in the UPS field, including a future Nobel Prize in Chemistry winner Aaron Ciechanover, proved that proteolysis was a burgeoning field. Major discoveries seemed to be emerging weekly in mammalian and fungal systems, and Genschik recognized that this could be a fruitful research direction for his future.

Attracted by the strength of its molecular biology research program, Genschik returned to Strasbourg and joined the Institut de Biologie Moléculaire de Plantes (IBMP) at the Centre National de la Recerche Scientifique as a group leader. From modest beginnings, with a doctoral student and a technician, Genschik expanded his research over the ensuing 23 years to make many profound discoveries centered on the impact of ubiquitin-dependent proteolysis machinery on plant development and environmental responses.

FILLING IN THE GAPS

Genschik’s appointment as a group leader at the IBMP in 1997 marked an exciting time that coincided with the beginning of the convergence of the UPS and phytohormone signaling fields. In particular, ubiquitin E3 ligases were becoming linked ever more closely to auxin and jasmonate signaling, although the precise mechanisms were still being elucidated. In a study published in the journal Cell, Genschik’s team showed that ethylene signaling is directly regulated by the UPS (Potuschak et al., 2003).

At that time, the mechanism by which the key transcription factor ETHYLENE INSENSITIVE3 (EIN3) promoted expression of ethylene-responsive genes was enigmatic because its transcription level is constant in the presence and absence of ethylene. Using yeast two-hybrid screening, Genschik’s team identified two F-box ubiquitin E3 ligases, EIN3 Binding F-box 1 (EBF1) and EBF2, that interact with EIN3 (Potuschak et al., 2003). Genetic and biochemical experiments showed that EIN3 is posttranslationally regulated by EBF1 and EBF2 and continuously committed for degradation until ethylene is present.

This opened a new question: how are EBF1 and EBF2 modulated upon ethylene perception to prevent constitutive EIN3 degradation? Further investigation by Genschik’s group found that the exoribonuclease XRN4/EIN5 posttranscriptionally regulates EBF1 and EBF2 levels (Potuschak et al., 2006). Collectively, these findings filled in a major gap in the ethylene signaling mechanism.

PLANT HIJACKING

The Genschik laboratory also made key findings into the mechanism by which Polerovirus suppresses host immunity to wreak havoc in plants. Poleroviruses use the effector protein P0 to inhibit the RNA silencing machinery and thereby nullify the plant’s defenses. However, the molecular mechanism by which this is accomplished was unknown. In collaboration with the group of Veronique Ziegler-Graff at IBMP, Genschik’s group found that the F-box domain of P0 is critical for virulence by forming an active Skp1-Cullin-F-box protein ubiquitin E3 ligase complex (Pazhouhandeh et al., 2006), suggesting that P0 hijacks the ubiquitin system to mediate degradation of plant defense proteins. They would later show that the degradation target is ARGONAUTE1 (AGO1), a central component of the RNA-Induced Silencing Complex whose inactivation compromises antiviral defenses (Bortolamiol et al., 2007). Interestingly, AGO1 clearance is not performed by the proteasome but by autophagy (Derrien et al., 2012; Michaeli et al., 2019). Furthermore, their work showed that plants have an endogenous, nonpathogenic route to clear defective AGO1s that have misloaded small RNAs (Derrien et al., 2012, 2018), a previously unrecognized mechanism that is conserved in animal cells.

Other notable findings have included the first evidence of a physical and functional link between CUL4-based E3 ligases and the Polycomb Repressive Complex 2 (Dumbliauskas et al. 2011; Pazhouhandeh et al., 2011), the function of CUL3-based E3 ligases in abscisic acid signaling (Lechner et al., 2011), the identification of ubiquitin ligases that permit cell cycle phase transitions (Genschik et al., 1998b; Capron et al., 2003; Noir et al., 2015), and a better understanding of the molecular mechanisms underlying plant stress tolerance conferred by DELLA proteins (Achard et al., 2008, 2009).

For these and his many other contributions to the field, Genschik was awarded the Gautheret Prize by the French Academy of Sciences in 2011 and became an elected member of the European Molecular Biology Organization in 2012. He has also maintained an active presence in the plant biology community, serving as a Scientific Director of the IBMP from 2005 to 2012 and acting as a senior editor for The Plant Cell since 2004. He also coordinated a multinational research project through the European Innovative Training Networks aimed at understanding the role of ubiquitin in plant hormone signaling and became a partner in the European Network of Excellence, which emphasized discovery of novel ubiquitin-dependent regulatory mechanisms in various model systems. Finally, Genschik was awarded a European Research Council Advanced Grant (2014 to 2019) to study the connection between autophagy and the RNA silencing machinery.

LOOKING FORWARD

Open questions in plant biology abound, and Genschik believes that one of the most pressing is understanding the huge complexity of posttranslational modifications in plants. Most cellular proteins encounter a multitude of posttranslational modifications within their lifespan. Even with an exhaustive cataloging of these modifications, the way in which these modifications regulate target function is contextual and can vary widely, thereby necessitating functional studies on a protein-by-protein basis.

In this regard, Genschik is keen on continuing to study ubiquitin and ubiquitin-related molecules, for which we still know little about the repertoire of modified substrates, complexity of these modifications (e.g., topology of ubiquitin chains), crosstalk between them, and how these modifications are read and interpreted in the cell. Many aspects of autophagy are also totally unexplored in plants (e.g., chlorophagy), and the number of autophagy receptors for these specialized pathways is still unknown. Genschik considers Richard Vierstra’s numerous pioneering discoveries on posttranslational modifications and the molecular mechanisms used by plants to selectively degrade intracellular proteins as a model in the field, especially where the work can be translated to mammalian systems or society as a whole.

At present, Genschik is focused on continuing to pursue multiple posttranslational modifications that regulate the RNA silencing machinery. Given that some are manipulated by pathogens to commandeer the host UPS, the research could one day provide effective strategies for fighting plant viral infections. Genschik predicts that this work will keep him occupied in the near and long term. Regardless of which direction his research takes him, one thing is certain: Genschik is the explorer he once dreamed of becoming.